The overall goal of the proposed renewal application is to identify features of distortion product otoacoustic emissions (DPOAEs) that will eventually improve clinical methods for the early detection of noise-induced hearing loss (NIHL), which is a major sensory disability suffered by military veterans, in particular. Toward this end, two special-purpose DPOAE measures, which we have termed augmented and residual DP-grams, along with related DPOAE level/phase (L/P) maps will be obtained in rabbits before and after standardized noise- exposure episodes. These DP-grams will test the notion that subtle post-noise changes in the DPOAE response space can be more sensitively identified when the f2- and basal-source generated DPOAEs are isolated by the use of an interference tone (IT) and vector-subtraction methods than they can be by the common standard DP-gram. During our prior studies, we unexpectedly (see Preliminary Studies) discovered that rabbits, and likely other common laboratory species, do not have 'reflection emissions' arising from the DPOAE frequency place (fdp) as do humans. However, inexplicably, they still exhibit vertical phasing banding, which is a cardinal feature of reflection emissions. Using suppression methods to obtain DPOAE L/P maps with and without an IT placed at various frequency locations within the L/P space, the existence of a basal cochlear source of DPOAEs was established. This basal source was revealed as a residual DPOAE obtained as the vector difference between the 'with-IT' and 'without IT' DPOAE L/P maps. The term 'augmented' maps or DP- grams was coined to describe these frequency functions when obtained with the IT present in that, under this condition, the basal source that 'fills in' or 'masks' the true extent of the damage pattern is removed. The discovery of the contaminating basal source promises to modify the hearing field's current knowledge concerning the fundamental processes underlying DPOAE generation by the cochlea, and may also lead to the development of DPOAE tests that more sensitively identify the onset stages of NIHL. Such tests may also be useful in veterans, who are clinic patients, often with significant preexisting hearing losses, in that higher-level primary tones can be used to increase the signal-to-noise ratio (SNR), while maintaining the test's sensitivity and frequency specificity. Three specific aims will be achieved to devise an optimal augmented DPOAE test and to evaluate its utility in detecting NIHL. The first aim under simulated NIHL conditions using a 4th tone tests the hypothesis that an optimal IT level for various primary-tone level combinations can be identified that most effectively removes (suppresses) the contaminating basal source, without affecting the f2 DPOAE source. The second specific aim tests the ability of the optimized 'augmented' (f2 source) and residual DP-gram (basal source) resulting from the experiments of Specific Aim #1 to identify the extent of cochlear damage under both temporary and permanent NIHL conditions in rabbits. The hypothesis tested here is that the augmented DP- gram will reveal in rabbits with noise-induced permanent threshold shifts a larger cochlear lesion than predicted by standard DP-grams. Finally, the third specific aim will test the ability of augmented DP-grams elicited by higher-level primary tones to uncover noise-damaged cochlear regions in veterans with NIHL. The notion examined here is that high-level primary tones will be more useful under conditions of preexisting hearing loss than conventional low-level primaries using the optimized augmented DP-gram by removing basal sources that come into play to obscure damaged cochlear regions as primary-tone levels are increased. Together, the combined experiments will provide a more complete understanding of the generation of DPOAEs, which will permit the creation of a useful clinical test for diagnosing and monitoring the development of NIHL.